The present invention relates to an arrangement for exhaust braking of a combustion engine by controlling passage of engine exhaust gases through the turbo for creating a back pressure in the exhaust passage.
Conventional exhaust brakes comprise in many cases a substantially disc-like damper arranged for pivoting within an exhaust line of a combustion engine. When the exhaust brake is activated, the damper is pivoted to a position in which it totally or partly blocks the flow of gases through the exhaust line. A resistance to the upward movement of the combustion engine's pistons during the combustion engine's exhaust phase is thus created, resulting in a braking effect. When the exhaust brake is not activated, the disc-like damper is pivoted to a position such that the exhaust gases can flow past the damper with substantially no resistance. The damper is arranged in the exhaust line upstream or downstream of the turbo unit of supercharged combustion engines.
Conventional turbo units comprise a turbine which drives a compressor which compresses air which is led to a combustion engine. The turbo unit also comprises a bypass line and a valve device (wastegate) which has the function of limiting the turbo unit's charging pressure. When the turbo unit's charging pressure reaches a predetermined value, the valve device opens so that the exhaust gases in the exhaust line are led through the bypass line and flow past the turbo unit. The turbo unit thereupon slows down and the compressor's charging pressure decreases. A disadvantage of conventional turbo units is that at low engine speeds they have difficulty in providing a desired charging pressure.
Turbo units with variable geometry known as VTG (variable turbine geometry) have become increasingly commonly used, particularly in vehicles powered by diesel engines. Such turbo units comprise flow elements in the form of vanes or guide rails by which the exhaust flow towards a turbine can be varied. The guide rails may be pivotable or else a movable guide railing may be used. The turbine's capacity can thus be optimised at different engine speeds. Such a control also makes it possible to regulate the flow of charge air to the combustion engine and the amount of recirculating exhaust gases EGR (exhaust gas recirculation). A known practice is the use of pivotable guide rails and movable guide railings to create a counterpressure in the exhaust line upstream of the turbo unit with the object of effecting exhaust braking of a vehicle. This pressure is related to the braking action obtained during the exhaust braking. When exhaust braking is to be effected, the pivotable guide rails or the guide railing are placed in such a position that they totally or almost totally shut off the flow of gases to the turbine. A problem with such an exhaust brake is that it is difficult to create a counterpressure with good precision, since very small positional changes of the guide rails or the guide railing result in great variations in counterpressure.